1. Field of the Invention
This invention relates to a nickel-base single crystal superalloy having high creep rupture strength at high temperatures, and to a process for its production. In particular, the invention relates to a nickel-base single crystal superalloy having high creep rupture strength at high temperatures which is suitable for use as a material for gas turbine blades in jet engines, power plants, etc.
2. Description of the Prior Art
Various technological fields now demand superalloys having high strength at elevated temperatures. For example, the output and thermal efficiency of gas turbines used in jet engines, power plants, etc. can be most effectively increased by elevating the temperature of the combustion gas, and for this purpose, blade materials having high creep rupture strength at high temperatures have been sought.
For many years, conventionally cast nickel-base superalloys having equiaxed grains have been used as superalloys having high creep rupture strength at elevated temperatures. Since, however, the conventionally cast nickel-base superalloys undergo grain boundary cracking in a direction perpendicular to their stress axis, there is a limit to the magnitude of their creep rupture strength at high temperatures. Methods have been developed to reduce grain boundary cracking perpendicular to the stress axis by adding grain boundary strengthening elements such as carbon, boron and zirconium, and resulted in some increase in creep rupture strength. With these methods, however, it has been impossible to prevent fully the grain boundary cracking in a direction perpendicular to the stress axis.
The directional solidification technique adapted to achieve the absence of a grain boundary perpendicular to the stress axis was developed in the early 1960's. Columnar grains obtained from a polycrystal alloy by using this technique were free from a grain boundary perpendicular to the stress axis and had greatly increased strength at high temperatures, but suffered from the defect of undergoing cracking along their grain boundaries during casting. This defect was later remedied by the addition of hafnium, and many columnar grained alloys, such as PWA 1422, have been developed and come into commercial acceptance since then.
J. J. Jackson et al. found that the creep properties of the directionally solidified alloy PWA 1422 could be greatly improved by solution heat treatment [J. J. Jackson et al., Met. Trans., 8A (1977), 1615]. Since, however, this alloy contains grain boundary strengthening elements, there was a limit in raising the solution temperature. This was because incipient melting occurred in the alloy at relatively low temperatures owing to the presence of the grain boundary strengthening elements. Thus, single crystal alloys which do not require grain boundary strengthening elements, namely contain no grain boundary have been developed.
Alloy 454 (a product of United Technologies Corporation) is one nickel-base single crystal superalloy now in practical use, and Alloy 203E (a product of United Technologies Corporation) and NASAIR 100 (a product of AiResearch Manufacturing Co.) are also investigated for practical application.
The present inventors, to the best of their knowledge, believe that these three single crystal alloys are closest to the single crystal alloy of the present invention in view of their creep rupture strengths at high temperatures.
Alloy 454 is a nickel-base single crystal superalloy disclosed in U.S. Pat. No. 4,209,348 and has a composition consisting of, by weight, 10% Cr, 4% W, 12% Ta, 5% Al, 1.5% Ti and 5% Co and the balance being Ni. The U.S. patent compares the creep rupture characteristics of alloy 454 with those of a commercial nickel-base columnar grained superalloy, PWA 1422, obtained by a prior art technique and containing Hf, C, B and Zr as grain boundary strengthening elements, and states: "Referring to Table II, it is apparent that under the test conditions employed, the invention alloy (454) was superior to the other alloys tested including SM 200, SM 200 (No, B, Zr), 444 and PWA 1422". The alloy composition of PWA 1422 containing grain boundary strengthening elements, as described in the above U.S. patent, consists of, by weight, 9% Cr, 12.0% W, 5% Al, 2.0% Ti, 10% Co, 2.0% Hf, 0.11% C, 0.015% B, 1.0% Cb, 0.10% Zr and the balance being Ni.
The alloys claimed in U.S. Pat. No. 4,209,348 including alloy 454 contain about 1 to about 2% of Ti and as small as about 3 to about 5% of W.
Alloy 203E mentioned above is a nickel-base single crystal superalloy described in U.S. Pat. No. 4,222,794 and has a composition consisting of, by weight, 5.0% Cr, 1.1% Ti, 2.0% Mo, 5.0% W, 3.0% Re, 6.5% Ta, 5.5% Al, 0.4% V and Ni being the balance. This U.S. patent compares the creep properties of Alloy 203E with those of known Alloy 454 and PWA 1422 in Table IV, and states: "Table IV shows that under test conditions of 1800.degree. F. and a 36 Ksi applied load Alloy 203 displays more than twice the performance of Alloy 454 and more than 8 times the life of Alloy 1422 in terms of time to 1% creep." Alloy 203E contains as many as 8 elements including expensive Re in addition to Ni, but the content of W is only 5%.
NASAIR 100 is a single crystal alloy having a composition consisting of, by weight, 9.0% Cr, 1.0% Mo, 10.5% W, 5.75% Al, 1.2% Ti, 3.3% Ta, &lt;0.01% C and the balance being Ni as indicated in T. E. Strangman, G. S. Hoppin III, C. M. Pipps, K. Harris, and R. E. Schwer, "Development of Exothermically Cast Single Crystal Mar-M247 and Derivative Alloys", pp. 215-224 in Proceedings of the 4th International Symposium on Superalloys, "Superalloys '80", Sept. 21-25, 1980, Seven Springs, Pa., U.S.A. Thus, NASAIR 100 is a nickel-base single crystal superalloy containing Mo and Ti as essential ingredients.
The three nickel-base single crystal superalloys considered to be closest to the alloy of this invention have higher creep rupture strengths at high temperatures than conventional other alloys, but the strengths at high temperatures of these three single crystal alloys are not entirely satisfactory for gas turbine uses. It is still desired in the art therefore to develop superalloys having higher strengths at high temperatures.